I noticed that when pine64 and the client device are too close (couple of feet) connectivity may not work at all, it is more stable if
1. there's at least 6+ feet between them
2. no major metal obstacles on the way (like in my case there were garage gate made of sheet metal) that was blocking connectivity.

I also tried external antenna for Pine64 wifi (5Dbi RP-SMA Antennas), which did not help much. WiFi works okay in the room
if pine is plugged in in one corner of the room and i connect to it from another corner.

ps. Interestingly enough, same garage gate is not an obstacle for RaspberryPI 3 wifi AP.
pps.I would look into syslog on pine64 to see if you can find some clues why connectivity is not established ...

Okay, I don't know what I did differently when I retried starting up the hotspot this time, other than running hostapd from the command line instead of as a daemon, but I was able to connect my phone to it with no problem. Speedtest.net shows the speed to be much slower than my internet speed, but my plans for the hotspot is mainly for transfer between local devices, so if that's fast enough I won't mess with it further.